Extracting data or commands from a noisy signal is a difficult task. Noisy data can occur when data is transmitted on a long cable or a radio link. Another situation in which data needs to be extracted from a noisy signal occurs in wireless power transmission. The term “wireless power” as utilized herein refers to the transmission of electrical energy from a power source to an electrical load without interconnecting wires. A common form of wireless power transmission utilizes two electromagnetically coupled coils to form a transformer through which power is transferred from the transmitting side to the receiving side. The transmitter may take the form of a pad having a coil embedded therein. The receiver may be built into a cellular telephone, for example, with a receiving side coil built into the back thereof. Although there is no direct contact between the transmitting and receiving coils, the close proximity of the coils and a judicious use of shielding allows for efficient transfer of energy from the transmitting side to the receiving side to operate a load, which may be a rechargeable battery being recharged through the system, for example.
FIG. 1 shows a block diagram of a prior art wireless power transmission system, generally as 100. The system comprises a transmitter side 102 and a receiver side 122. The transmitter side 102 comprises a circuit 104 for rectifying an AC input into a DC voltage which is fed into a power stage 106 for generating a high frequency signal. The high frequency signal is coupled across a transformer 120 to the receiver side 122. The power stage 106 is controlled by a controller 108 which contains a threshold setting circuit 110. The threshold setting circuit 110 could be external to the controller 108. The power stage 106 and the controller 108 could be combined into a single integrated circuit. The receiver side 122 comprises a rectifier circuit 124 to output a DC voltage to a voltage conditioning circuit 126 which is operated by the receiving controller 128 to supply power to a load 130, which may be a rechargeable battery being recharged by the system, for example.
As shown in FIG. 1, power flows from left to right from the transmitter to the receiver and communications flow from right to left from the receiver to the transmitter. The communication signals may be command signals to adjust the power level from the transmitter or other parameters, for example. The communication signals may be generated by coupling a resistor or capacitor across the receiving coil to generate signals which can be recognized by the controller on the transmitting side. These low level signals are noisy because of the noise generated by the power transmission portion of the system.
FIG. 2 shows a prior art circuit for setting a voltage threshold level for extracting the data or commands from this noisy signal, generally as 110. The data or command signals are applied at terminal 202 and charge the capacitor 206 via resistor 204. A comparator 208 receives this threshold voltage and utilizes it to extract the data from a noisy signal, as is well known in the art. A problem with the circuit as illustrated in FIG. 3, is that the data or commands in wireless power transmission systems trends occur at intervals, so that the voltage across capacitor 206 is lost. Therefore, every time data is sent, the capacitor must be recharged before the appropriate threshold is generated. This is shown in FIG. 3 where data transmission starts after idle and the threshold signal Thr starts charging with the first data pulse. The threshold does not reach a value allowing the data to be retrieved until several pulses have passed. Thus, data in those first pulses is lost. This is illustrated in greater detail in FIGS. 4, 5 and 6. In these figures, the axis is time in milliseconds and the ordinate is volts. FIG. 4 illustrates the incoming data 402. FIG. 5 illustrates the charging of the threshold generating capacitor at 502 and FIG. 6 shows the data loss 602 from the pulses 604. A typical circuit might have a resistor of 30 mega ohms and a capacitor of 200 pF, for example. Increasing the size of the capacitor or resistor would reduce the charge lost when the circuit is at idle, but having large capacitors or resistors on a chip requires a large area on the chip and having external components increases the cost and size of the circuit.
Another known technique for extracting the threshold value is to utilize an ADC circuit driving a microprocessor and extracting the data utilizing a software routine (not shown). This is an expensive solution.
Thus, there is the need for a low cost, highly integratable threshold circuit that reaches its full threshold value quickly so that no data is lost.